This invention relates to the art of materials science and, more particularly, to nonmetallic materials and powder metallurgy.
Ceramic materials have certain outstanding properties, such as high temperature strength, corrosion resistance, low density, and low thermal expansion, which make them attractive materials for high temperature applications. Silicon nitride (Si.sub.3 N.sub.4) is a ceramic which has a desirable combination of properties for structural use at high temperatures. These are high strength, oxidation resistance, and resistance to thermal shock. However, the fracture toughness (resistance to fracture) of Si.sub.3 N.sub.4 at both room temperature and elevated temperature is only moderate. Also, it is susceptible to slow crack growth at temperatures above about 1200.degree. C.; when cracks develop, strength is degraded. Slow crack growth also is the cause of Si.sub.3 N.sub.4 having a relatively short stress rupture lifetime. Monolithic Si.sub.3 N.sub.4 is currently being tested in a fleet of automobiles as a turbocharger rotor material. Operating temperature is limited to 1000.degree. C., which is an undesirably low temperature for this application. If the deficiencies of Si.sub.3 N.sub.4 can be overcome, it has the potential to become an important high temperature structural material.
There is a class of materials which offers the advantages of a ceramic and certain of the beneficial mechanical characteristics of a metal. These materials are intermetallics, which at high temperatures have the excellent properties of a ceramic, but also behave mechanically like a metal in that they show yielding and stress-relieving characteristics. Molybdenum disilicide (MoSi.sub.2) is an intermetallic compound which has potential for structural use in oxidizing environments at high temperatures. It has a melting point of 2030.degree. C. and its oxidation resistance at high temperatures is excellent, since it forms a protective SiO.sub.2 layer. Mechanically, MoSi.sub.2 behaves as a metal at high temperatures since it undergoes a brittle-to-ductile transition at about 900-1000.degree. C. Thus, MoSi.sub.2 has a stress relieving characteristic at high temperatures. These characteristics of MoSi.sub.2 point toward its use in combination with Si.sub.3 N.sub.4. Reinforcement of Si.sub.3 N.sub.4 with MoSi.sub.2 particles may significantly improve the elevated temperature mechanical properties as compared to pure Si.sub.3 N.sub.4. It is expected that Si.sub.3 N.sub.4 --MoSi.sub.2 composites will possess improved strength, fracture toughness and resistance to crack growth at high temperatures. It is believed that above 1000.degree. C. MoSi.sub.2 particles will restrain initiation and propagation of brittle cracks in the ceramic matrix by means of plastic deformation energy absorption mechanisms. Such mechanisms for aluminum metal particles in glass have been shown to improve fracture toughness of the glass composite by a factor of seven. Also, because the MoSi.sub.2 particles are a second phase reinforcement, the possibility exists for improvements in low temperature fracture toughness through toughening mechanisms such as crack deflection even though MoSi.sub.2 is brittle at room temperature. MoSi.sub.2 is thermodynamically stable and chemically stable with Si.sub.3 N.sub.4 at elevated temperature. Examples of immediate applications for the inventive materials are vehicular engine components such as turbocharger rotors, valves, swirl chambers, rocker arm tips, piston pins, and tappet faces.
Because the room temperature electrical conductivity of MoSi.sub.2 is relatively high, it may be possible to use electrodischarge machining of the inventive composites. This method of machining is significantly less expensive than the diamond machining process which is presently used for silicon nitride. Also, though Si.sub.3 N.sub.4 will not couple to 2.45 GHz microwave radiation at room temperature, it is expected that the inventive composites will do so, so that microwave processing can be used in their manufacture.